Structural Steel Defects Flashcards
What is another name for Embedded Steel Frame Corrosion and what is this defect?
Otherwise known as ‘Regent Street Disease’. The steel frame corrodes causes cracks or displacement of the encasing façade.
What causes Regent Street Disease?
Expansive rusting of the corroding steel can crack or displace the encasing façade. Moisture can penetrate through porous cladding materials, open or degraded mortar joints, or faulty or degraded services such as cracked rainwater downpipes and gutters.
What are the remedies for Regent Street Disease?
Remove the water source asap. If in early stages of corrosion, cathodic protection can be installed.
If in latter stages, affected areas of the facade have to be removed and replaced after the rust has been removed adn the steel has been treated with a corrosion inhibitor.
What is carbonation?
An electrochemical process that affects all steel reinforced concrete structures.
How can you diagnose carbonation?
Visual inspection can identify cracking along reinforcement lines and spalling of the concrete cover, exposing the rusting steel reinforcement beneath.
It is of more concern on exposed, poor, shallow concrete covers.
Carbonation depth can be determined by using phenolphthalein indicator or the concrete can be lab tested using petrographic analysis.
What causes concrete carbonation?
Ingress of atmospheric carbon dioxide reacting with water forming a carbonic acid which neutralises the alkalinity in the concrete. This breaks down the passivity layer around the reinforcement causing it to oxidise.
Although all concrete suffers from carbonation, the rate at which is occurs can e affected by atmospheric exposure to pollutants, the type of aggregate used and the porosity/density of the concrete.
What is the remedy for concrete carbonation?
Significant instances should be referred to a structural engineer for further investigation.
Otherwise, take the affected steel back to a metallic surface, treat with a corrosion inhibitor and then replace the concrete cover or a polymer-containing mortar.
What is HAC?
High Alumina Cement is cement that was used between 1954 and 1976 (when it was banned). Unlike Portland cement that is made of calcium silicates, HAC is composed of calcium aluminates. It was developed as it cures very quickly making construction quick and more efficient.
How can you diagnose HAC?
Concrete should be tested to verify the presence of HAC, but visually, it is normally a light brown colour.
What causes issues with HAC?
Moisture hydrates HAC (conversion) to form hydrated calcium aluminates, a denser but more porous and weaker cement.
What are the remedies for HAC?
A structural engineer should undertake a strength assessment. Most structures have a sufficient reserve of strength even after conversion but it should be monitored carefully, especially in moist and hot conditions.
What is concrete cancer?
This is another name for the alkali-silica reaction. It occurs over time between highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates. Given sufficient moisture, it forms a soluble and viscous gel that expands the aggregate, increasing its volume, causing cracks.
How can you diagnose concrete cancer?
Visual inspection identifies mapped cracking (crazing), surface spalling and loss of strength.
Sodium oxide equivalent scale is a chemical test to test alkali content.
Concrete core samples can be examined using a petrographic microscope.
Polished sections can be analysed using x-ray microanalysis
What is the remedy for concrete cancer?
Remove the source of water. The amount of alkali metal and reactive silica content in concrete should be limited in design.
In less severe cases, localised repair is possible, including drying the structure and the installation of a waterproof membrane.
In severe cases, structural problems can develop until demolition is required.
How is phenolphthalein indicator used?
Poured over suspected carbonated concrete. If carbonated, will turn colourless, if not, will turn violet because alkali.
